1. Field of the Invention
The present invention relates to living random (meth)acrylic copolymers, to copolymers and block copolymers prepared therefrom and to the corresponding processes for the production thereof via anionic copolymerization.
2. Description of the Prior Art
The provision of a completely living system for the copolymerization of (meth)acrylic monomers, in particular of (meth)acrylic esters, has long eluded the art of polymer synthesis. Partial solutions to this problem has been provided by the discoveries of group-transfer polymerization (Webster et al, New Methods for Polymerization Synthesis, Mijs, W. S. ed., page 1, Plenum Press, New York (1992), of metal-free anionic polymerization (Reetz et al, Angew. Chem. Int. Ed. Engl., 27, 1373 (1988)) and of the conventional carbanionic initiator modified with a ligand (Fayt et al, Macromolecules, 20, 1442 (1987); Varshney et al, ibid., 23, 2618 (1990); Varshney et al, ibid., 25, 4457 (1992); U.S. Pat. No. 4,056,580; Vicek et al, J. Makromol. Chem., Rapid Commun., 13, 163 (1992); Kitayama et al, Makromol. Chem., Supp. 1989, 15, 167; Ballard et al, Macromolecules, 25, 5907 (1992)). It has evolved that only this latter technique, were it to succeed, will provide a general mechanism enabling other monomers to be easily combined, among others the vinyl monomers and cyclic ethers, with (meth)acrylic monomers.
Although "living" anionic homopolymerization and "living" anionic block copolymerization of methacrylic and acrylic monomers (especially methacrylates and acrylates) have been recently developed to a fairly advanced degree, by virtue of the use of a number of effective ligands, living anionic random copolymerization of mixtures of these methacrylic and acrylic monomers is to date still very difficult. Because of the great difference in reactivity of the respective monomers, the group-transfer polymerization (GTP) technique does not permit the copolymerization of mixtures of acrylates and methacrylates (Webster et al, op. cit.). The same limitation is encountered in metal-free anionic polymerization (Reetz et al., op. cit.).
It has been demonstrated that the use of ligands forming mixed aggregates with the active center, such as LiCl, is an effective route for preventing the progress of the anionic polymerization from being perturbed by secondary transfer and termination reactions (Fayt et al, Macromolecules, 20, 1442 (1987); Varshney et al, ibid., 23, 2618 (1990); Varshney et al, ibid., 25, 4457 (1992)) for shifting the equilibria between various levels of aggregation of the active center towards a mixed aggregate, as indicated above (Kunkel et al, Makromol. Chem., Makromol. Symp., 60, 315 (1992)). Utilizing this technique block copolymers of methyl methacrylic and tert-butyl acrylate have been synthesized with a predictable molecular weight and a narrow dispersity, whatever the monomer polymerized first (Varshney et al, Macromolecules, 24, 4997 (1991)). However, completely to the contrary, a simultaneous random or "tapered" (namely, producing a polymer whose composition along the backbone chain decreases in respect of a given monomer) copolymerization of these monomers does not proceed as expected and provides very mediocre results in respect of conversion and of distribution of the molecular weights, because of the presence of a selective nucleophilic attack on a highly sensitive methyl ester group by a sterically relatively unhindered poly(tert-butyl acrylate) anion, namely, of an intramolecular termination reaction via a "back-biting" mechanism (Jacobs et al, Macromolecules, 23, 4024 (1990)).
This "back-biting" mechanism can be represented as follows in the case of the copolymerization of methyl methacrylate with tert-butyl acrylate, it being possible for a tert-butyl acrylate endgroup to react with an antepenultimate methyl methacrylate residue along the polymer chain, i.e.: ##STR1##
Thus, in the presence of LiCl as an initiator in tetrahydrofuran at -78.degree. C., random copolymerization entailing (meth)acrylic monomers does not succeed.